The present invention relates to a technology for controlling the operation for positioning and tracing a member to a target position. More particularly, the present invention relates to a radial tracking control apparatus or an axial tracking control (focus control) apparatus in an optical disk apparatus or a positioning control apparatus for positioning and tracing a transducer for recording and reading a signal of a laser beam spot, a magnetic head onto a disc-shaped recording medium, such as a tracking apparatus in a magnetic disk apparatus, and a positioning control method.
Conventionally, optical disk apparatuses for recording or reading information by irradiating laser beams onto a disk-shaped recording medium are widely used. As the above-mentioned optical disk apparatuses, there are a CD, a CD-R, a CD-RW, a DVD-ROM, etc. These optical disk apparatuses require the beam tracking to a track for recording information with high accuracy so as to improve the density for recording the information. Further, these optical disk apparatuses require the focus control (axial tracking) with high accuracy in accordance with the vertical deviation of an optical-disk surface.
In general, the improvement in accuracy of the tracking control uses a method for improving a loop gain of a control system and increasing a response frequency of the control-system loop. However, since characteristics of a mechanical system for moving a moving member are limited, the above-mentioned method cannot ensure sufficient accuracy.
In order to solve the above problem, a position error is compressed by using the regularity of the vertical deviation of the optical-disk or the eccentric of a recording track of the disk-shaped recording medium. That is, the rotation of the optical disk causes the position shift on the optical-disk or the radial movement of the optical-disk, and a position shift component or a radial-movement component is approximately synchronized with the rotation of the optical disk. Therefore, the tracking of the moving member (laser beam spot) to a target member (information recording position on the optical disk) can be improved by a position offset signal from one to several numbers of rotations by using the periodicity of the position shift of the information recording position on the optical disk.
As disclosed in Japanese Examined Patent Application Publication No. 60-57085, a “positioning control apparatus” is proposed (hereinafter, referred to as a first conventional art). According to the first conventional art, the positioning control apparatus comprises a signal delay unit for adding and accumulating a position error signal synchronously with a rotational period every position shift with a predetermined period. Consequently, the position error signal is added and is inputted to the signal delay unit, and a moving member is driven based on a signal obtained by adding the position error signal.
Herein, it is defined that a basic control unit comprises a position detector for detecting a relative position error between a target member and the moving member, a compensating unit for performing at least one of compensation for stabilization of a control loop and compensation for offset, and a drive unit for driving the moving member, which are serially combined.
According to the first conventional art, a transfer function of the basic control unit is expressed by G(s) and a periodic position shift of the target member is expressed by Xi. In this case, if the periodic position shift Xi is repeated n times, a relative position error Xe is expressed by [Xe=Xi/{1+G(s)}n which means that an output of delay means comes close to Xi/G(s).
More specifically, the relative position error makes an approach to zero within a frequency band having an absolute of [1+G(s)], which is larger than 1. Therefore, a signal for tracking a positioning member to the periodic position shift is applied almost by the output of the signal delay unit. Thus, the tracking can excessively be improved without so increasing the gain of the basic control unit (absolute of G(s)) or the response frequency.
A “positioning control apparatus and a positioning control method” are obtained by further improving the first conventional art as disclosed in Japanese Patent Application No. 2001-030525 (hereinafter, referred to as a second conventional art).
According to the second conventional art, the positioning control apparatus comprises a signal delay unit which adds and accumulates a relative error between a target position which is periodically shifted and a moving member. An output of the signal delay unit is added to a position error signal at this time. The moving member is driven based on the addition signal and a signal obtained by filtering processing of the output of the signal delay unit of a filter having specified characteristics.
According to the second conventional art, it is assumed that a transfer function of a filter is expressed by F(s). When the periodic position shift is repeated n times, a relative position error Xe is expressed by [Xe(n)={(1−G(s)F(s))n−1/(1+G(s))n}−Xi].
By setting proper filter characteristics F(s) in such a manner that a value of G(s)F(s) approaches 1, even if a value of [1+G(s)] is smaller than 1 and [1−G(s)F(s)] as a numerator is further smaller, the remaining position error Xe(n) can come close to zero.
However, the first and second conventional arts can be applied to an optical disk apparatus using a CAV (Constant Angular Velocity) format as a recording format because the rotational period of the optical disk apparatus is approximately constant, irrespective of the position of the laser beam spot in the radius direction of the optical disk apparatus. On the other hand, although the first and second conventional arts can be applied to an optical disk apparatus using a CLV (Constant Linear Velocity) format as the recording format, there is a drawback that the increase in accumulated phase offsets of the position error signal, relative to a rotational phase of the optical disk apparatus, causes the deterioration in compression performance of the position error signal in accordance with the movement of the laser light beam spot in the radius direction of the optical disk apparatus due to the change in rotational period of the optical disk apparatus depending on the position of the light beam spot in the radius direction, of the optical disk apparatus.
Further, the first and second conventional arts have a problem that when the position error signal is not repeatedly generated, a signal for driving the moving member does not necessarily compress the position error. In particular, when a phase of the position error signal is inverted, the position error is further deteriorated after one period.
For example, in consideration of a initial response of the position error signal according to the first conventional art, it is assumed that the amounts of position shift of the target member are Xi(1), Xi(2), and Xi(3) at first to third periods of the optical disk apparatus after starting the positioning control according to the first conventional art. Further, it is assumed that the relative position errors are Xe(1), Xe(2), and Xe(3) at the first to third periods. Since the position error signals are not added and accumulated at the first period of the rotation of the optical disk apparatus, the relative position error is expressed by the following formula, similarly to the case of using no first conventional art.Xe(1)=Xi(1)/(1+G(s))  (B1)
The position error signals at the first and second periods are added and accumulated into the signal delay unit at the second and third periods of the rotation of the optical disk apparatus. Therefore, formulae (B2) and (B3) are established.
                              Xe          ⁡                      (            2            )                          =                ⁢                              {                                          Xi                ⁡                                  (                  2                  )                                            -                                                Xi                  ⁡                                      (                    1                    )                                                  ·                                                      G                    ⁡                                          (                      s                      )                                                        /                                      (                                          1                      +                                              G                        ⁡                                                  (                          s                          )                                                                                      )                                                                        }                    /                      (                          1              +                              G                ⁡                                  (                  s                  )                                                      )                                              (        B2        )                                                                                    Xe                ⁡                                  (                  3                  )                                            =                            ⁢                              {                                                      Xi                    ⁡                                          (                      3                      )                                                        -                                                            Xi                      ⁡                                              (                        2                        )                                                              ·                                                                  G                        ⁡                                                  (                          s                          )                                                                    /                                              (                                                  1                          +                                                      G                            ⁡                                                          (                              s                              )                                                                                                      )                                                                              -                                                            Xi                      ⁡                                              (                        2                        )                                                              ·                                                                  G                        ⁡                                                  (                          s                          )                                                                    /                                                                                                                                                                              ⁢                                                                            (                                              1                        +                                                  G                          ⁡                                                      (                            s                            )                                                                                              )                                        2                                    }                                            /                              (                                  1                  +                                      G                    ⁡                                          (                      s                      )                                                                      )                                                                        (        B3        )            
If the amounts of position shift of the target member are equal at the first to third periods of the rotation of the optical disk apparatus, in other words, if the period of the position shift of the target member is maintained and a formula of [Xi(1)=Xi(2)=Xi(3)=Xi] is established, the formula (B2) is expressed by [Xe(2)=Xi/(1+G(s))2] and the formula (B3) is expressed by [Xe(3)=Xi/(1+G(s))3]. In other words, in a frequency range with a larger gain of [1+G(s)] than 1, the relative position error is compressed every period of the rotation of the optical disk apparatus.
However, if the phase of the position shift of the target member Is inverted at the second period of the disk rotation, namely, if the periodicity of the position shift of the target member is not maintained and a relationship of [Xi(1)=Xi(3)=Xi] and a relationship of [Xi(2)=−Xi] are established, the above formula (B3) is as follows.Xe(3)=Xi[{G(s)/(1+G(s))}2+1]/(1+G(s))  (B4)
In the case of comparing the formula (B1) with the formula (B4), if the phase of the position shift is inversed at the second period, it will obviously be understood that in a frequency range with a larger gain of [1+G(s)] than 1, the amount of compression of the position error is deteriorated up to the half at the third period. Based on the formulae (B1) to (B4), the relative position error is represented every period of the disk rotation. Similarly, if the period of the position shift of the target member is not maintained only in the case of a part of the rotational phase of the disk rotation, the relative position error is partly deteriorated in the case of the part of the rotational phase of the disk after one period.
Although the phase inverse of the position shift of the target member has been described based on the formulae (1) to (4), equivalently, mixing of a signal asynchronous with the rotation of the optical disk to the amount of movement of the moving member also deteriorates the amount of compression of the position error. However, the amount of compression of the position error is decreased as the phase offset approaches the inverse phase though the phase is not inversed. Similarly, according to the second conventional art, in the positioning control apparatus comprising the means for adding and accumulating the position error signal, the amount of compression of the position error is deteriorated when the position error signal is not repeatedly generated.
Just after the control loop is closed, the period of the position error signal is not necessarily maintained. Specifically speaking, just after the control loop is closed, a response of the position error signal almost has frequency characteristics of the closed loop in the basic control unit. It does not necessarily reflect the periodicity of the position shift. Further, when an asynchronous disturbance oscillation is applied to the tracking controlling apparatus, the periodicity of the position error signal is not maintained.
According to the first and second conventional arts, components asynchronous with the frequency caused by repeating the periodic position shift deteriorate a compression ratio of the position error signal. For example, when a disturbance asynchronous with the frequency caused by repeating the periodic position shift is continuously applied, the signal components caused by the asynchronous disturbance might deteriorate the position error signal.
That is, according to the first and second conventional arts, the tracking can be improved without extremely increasing the gain of the basic control unit or the response frequency of the basic control unit. In addition, the position error having a frequency higher than a cut-off frequency of the basic control unit can be suppressed. However, the first and second conventional arts require the improvement.
Compression characteristics Gc1(s) of the position error signal are expressed by the following formula (C1) when the position error signal is not added and accumulated. Compression characteristics Gc2(s) of the position error signal are expressed by the following formula (C2) when the position error signal is added and accumulated. The compression characteristics of the position error signal are defined by transfer characteristics in a signal route to position error signal from the position shift of the target member.Gc1(s)=1/{1+G(s)}  (C1)Gc2(s)=1/{1+G(s)/(1−e−Ls)}  (C2)where s=j×2πf, L: the amount of signal delay in the signal delay unit (the amount of delay: time), J: imaginary-number unit, and f: frequency.
The formula (C2) has a maximum value near an intermediate period of the amount of delay. In other words, when the frequency f in the formula (C2) has the maximum value near a frequency satisfying [f0=(2k+1)/2L (k=0, 1, 2, 3 . . . ). The formula (C2) is expressed by the formula (C3) when the frequency f=f0.Gc2′(s)=1/{1+G(s)/2}  (C3)
In the case of comparing the formula (C1) with the formula (C3), within a frequency band having G(s) substantially larger than 1 (namely, frequency band having G(s) substantially smaller than the cut-off frequency of the basic control unit), the amount of compression is deteriorated to the half according to the first and second conventional arts.
In other words, according to the first and second conventional arts, in the case of the position shift of the target member at the intermediate period of the amount of delay, the amount of compression of the position error signal is deteriorated to the half.
In the optical disk apparatus, since the main frequency component of the position error signal is a rotational-period frequency of the optical disk apparatus or a harmonic component of the rotational period, the regularity of repetition of the position error signals can almost be held. However, the disturbance oscillation asynchronous with the rotational period might continuously be multiplied to the position error signal.
As one example of the asynchronous disturbance oscillation, there is a beat oscillation caused by a spindle motor for rotating and driving the optical disk. Further, as another example, there is an optical-head oscillation caused by a feed mechanism of an optical head by stick slip when moving the laser beam spot in the radius direction of the optical disk apparatus. Incidentally, the stick slip means the transition from static friction to dynamic friction or the transition reverse thereto.
The asynchronous disturbance oscillation is not mainly caused in the position shift or the radial movement of the optical-disk. However, it is mainly caused in the position error signal after tracking with high accuracy in accordance with the component synchronous with the rotation according to the first and second conventional arts. Therefore, the asynchronous disturbance oscillation degrades higher tracking accuracy.
In the optical disk apparatus, the asynchronous disturbance oscillation substantially exists as a lower frequency component, as compared with, mainly, the cut-off frequency in the basic control unit. Therefore, according to the first and second conventional arts, preferably, the remaining position error signal can be obtained without adding and accumulating the position error signal at the frequency band lower than the cut-off frequency in the basic control unit.